1. Field of the Invention
This invention relates to an objective lens driving device employed in an optical disc recording/reproducing apparatus, such as an optical disc player, and an optical pickup device employing the objective lens driving device.
2. Description of related Art
In an optical disc recording/reproducing apparatus, such as an optical disc player, employing an optical disc as a recording medium, an objective lens driving device is employed for displacing an objective lens in a direction parallel to its optical axis and within a plane at right angles to the optical axis. The objective lens is adapted for collecting a light beam radiated from a light source, such as a semiconductor laser, and for radiating the collected light beam to the optical disc.
In the optical disc driving device, the objective lens is displaced, responsive to the focusing error signals and the tracking error signals, in a direction parallel to the optical disc of the objective lens and in a plane normal to the optical axis, whereby a light beam radiated by the objective lens on a signal recording surface of the optical disc rotated by the disc rotating driving device is focused on the signal recording surface of the optical disc, and the light beam is caused to follow the recording track formed on the optical disc.
FIGS. 1 and 2 show a typical conventional objective lens driving device.
The objective lens driving device includes a bobbin 2 having an objective lens 1 mounted at its one end. The bobbin 2 is supported in a cantilevered fashion on a stationary support member 8, mounted on a yoke 5 constituting a magnetic circuit unit 4, with the aid of four resilient supporting members 3, such as wires.
The bobbin 2, carrying the objective lens, has a center aperture 7 extending along the optical axis of the objective lens 1. Within the aperture 7 is disposed a focusing coil 8 in the form of a square tube. On an outer lateral side of the focusing coil 8 is bonded a tracking coil 9 made up of planar rectangular coils 9a, 9b.
On each lateral side of the bobbin 2 is mounted a relaying printed circuit board 10 to which are electrically connected coil terminals 8a, 9a lead out respectively from the focusing coil 8 and the tracking coil 9. On the printed circuit board 10 is formed a connecting pattern 10a to which an end 3a of the resilient supporting member 3, formed of an electrically conductive material for functioning as a power feed line, is electrically and mechanically connected using an electrically conductive adhesive, such as solder.
A pair of such resilient supporting members 3, each supporting the bobbin 2 at its one end 3a, are arranged on each lateral surface of the bobbin 2 in parallel with each other. The resilient supporting members 3 are fixedly supported by a stationary supporting member 8 mounted on the yoke 5 by having the other ends passed through through-holes 11 bored at respective corners of the stationary supporting member 8. The other ends 3b of the resilient supporting members 3, protruded via the through-holes 11 of the resilient supporting members 3, are electrically connected to a driving control circuit, not shown, designed for displacing the objective lens 1.
The objective lens 1, mounted on the bobbin 2, supported in a cantilevered fashion by the stationary supporting member 6 by each two resilient supporting members 3 on both lateral sides of the bobbin 2, may be moved in a direction parallel to the optical axis within a plane normal to the optical axis of the objective lens 1, with the resilient supporting members 3 as the deflecting members, as indicated by arrows F and T in FIG. 1, respectively.
The yoke 5, on which the stationary supporting member 6 is mounted, is formed with a pair of upstanding pieces 12, 13 facing each other. On the surface of the piece 12 facing the opposite side piece 13 is affixed a magnet 14 constituting a magnetic circuit unit 4.
The stationary supporting member 8 supporting the bobbin 2 via the plural resilient supporting members 3 is mounted on the upper surface on the opposite end of the yoke 5 for making up the objective lens driving unit. At this time, the upstanding pieces 12, 13 are intruded into the aperture 7 of the bobbin 2, with the focusing coil 8 and the tracking coil 9 in-between, as shown in FIG. 1. The focusing coil 8 and the tracking coil 9 are arranged at the position of being interlinked with the magnetic flux emanating from the magnet 14 towards the opposite side upstanding piece 13.
With the above-described objective lens driving device, when the control current corresponding to the focusing error signal is supplied from the driving control circuit via the resilient electrically conductive supporting members 3 to the focusing coil 8, there is generated a driving force of displacing the bobbin 2 in a direction parallel to the optical axis of the objective lens 1 by the interaction of the control current with the magnetic flux of the magnetic circuit 4. The bobbin 2 is displaced in the focusing direction, that is the direction parallel to the optical axis of the objective lens 1, as indicated by arrow F in FIG. 48, while elastically displacing the resilient supporting members 3. By such displacement of the bobbin 2, the objective lens 1 mounted on the bobbin 2 is also displaced in the same direction for performing focusing control.
When the control current corresponding to the tracking error signal is supplied from the driving control circuit via the resilient electrically conductive supporting members 3 to the tracking coil 9, there is generated a driving force of displacing the bobbin 2 in a direction normal to the optical axis of the objective lens 1 by the interaction of the control current with the magnetic flux of the magnetic circuit 4. The bobbin 2 is displaced in the tracking direction, that is the direction normal to the optical axis of the objective lens 1, as indicated by arrow T in FIG. 1, while elastically displacing the resilient supporting members 3. By such displacement of the bobbin 2, the objective lens i mounted on the bobbin 2 is also displaced in the same direction for performing tracking control.
With the above-described objective lens driving device, the relaying printed circuit board 10 is arranged on each side of the bobbin 2 and coil ends 8a, 9a of the focusing coil 8 and the tracking coil 9 are connected to the printed circuit board 10. The resilient supporting members 3, each having its one end to the printed circuit board 10, are provided for supplying the control current to the focusing coil 8 or to the tracking coil 9. There is also known an arrangement in which the control current is fed via a flexible printed circuit board 16 to the focusing coil 8 or to the tracking coil 9.
With such objective lens driving device, shown in FIG. 3, the flexible printed circuit board 18, connected to a control circuit, has its one end connected to the upper end face of the bobbin 2, and coil ends 8a, 9a of the focusing coil 8 and the tracking coil 9 are connected to a connection pattern 16a formed at one end of the flexible printed circuit board 16. By employing the flexible printed circuit board 16, the resilient supporting member 3 need not be formed of an electrically conductive material, but may be formed of a material exhibiting desired properties, such as desired degree of resiliency.
On the other hand, if the flexible printed circuit board 16 is employed, there is no necessity of providing the relaying printed circuit board 10 on the bobbin 2, so that the resilient supporting member 3 has its one end 3a directly mounted on the bobbin 2 via engaging supports 17 formed on either sides of the bobbin 2.
The focusing coil 8 and the tracking coil 9, employed in the above-described objective lens driving device, are formed by winding a wire. The focusing coil 8 is wound in a square tube from a single wire and the connecting coil end 8a is led from each of upper and lower ends thereof, as shown in FIG. 2. The tracking coil 9 is formed by winding a single wire for providing two rectangular coils 9b, 9c side-by-side and the connecting coil terminal 9a is led from a side of each of the coils 9b, 9c, as shown in FIG. 2. The tracking coil 9 is affixed and unified to a lateral side of the tubular focusing coil 8 as shown in FIG. 2. The focusing coil 8, thus carrying the tracking coil 9, is directly affixed to the bobbin 2 by having its side opposite to the side carrying the tracking coil bonded to the inner wall of the aperture 7, as shown in FIG. 1.
The coil ends 8a, 9a, led from the focusing coil 8 and the tracking coil 9, are electrically connected to the relaying printed circuit board 10 or to the flexible printed circuit board 16 using an electrically conductive adhesive, such as solder. Thus the operation of connecting the coil ends 8a, 9a is necessary to perform during the assembly operation of the objective lens driving device, thus lowering the assembling efficiency.
Besides, it is necessary to prevent the coil ends 8a, 9a from becoming loosened on the bobbin 2 when connecting the coil ends. If the coil end 8a, 9a are loosened, there is the risk that the objective lens i cannot be driven accurately responsive to the control current due to inadvertent movement of the coil ends 8a, 9a during displacement of the objective lens 1. If, with the objective lens driving device, the objective lens i is to be moved along the optical axis and in a direction normal thereto in stability and with high response to the driving current supplied to the focusing coil 8 and the tracking coil 9, the center of gravity P of the bobbin 2 mounting the objective lens 1 as a movable part needs to be correctly coincident with the center of generation of the driving force of driving and displacing the bobbin 2.
With the above-described objective lens driving device, the driving force of displacing the objective lens 1 in a direction parallel to the optical axis of the objective lens 1 is produced by the interaction of the control current flowing through a coil portion 8b extending in a direction normal to the optical axis of the objective lens 1, that is in a direction defined by the lateral surface section of the focusing coil 8 disposed between the upstanding pieces 12, 13, and the magnetic flux radiated by the magnet 14 and directed from the piece 12 towards the piece 13 for being interlinked with the coil portion 8b, as shown in FIGS. 1 and 5. The driving force of displacing the objective lens 1 in the tracking direction, that is in a planar direction normal to the optical axis of the objective lens 1, is produced by the interaction of the control current flowing through linear sections 19a, 19b of the rectangular coils 9a, 9b of the tracking coil 9 extending parallel to the optical axis of the objective lens 1 and the magnetic flux radiated from the magnet 14 and directed from the upstanding piece 12 towards the opposite upstanding piece 13 for being interlinked with the linear sections 19a, 19b. The coil portions 9a, 9b are disposed on the side of the focusing coil lying between the upstanding pieces 12 and 13, as shown in FIGS.1 and 4.
The coil portions 9a, 9b of the tracking coil 9 are connected so that the current will flow in the same direction of linear portions 19a, 19b lying between the upstanding portions 19a, 19b.
For accurately generating the driving force for the objective lens 1 in a direction parallel to the optical axis, the coil portion 8b of the focusing coil lying between the upstanding pieces 12, 13 needs to be coincident with a line Y--Y', shown in FIG. 4, which represents the center of the magnetic gap defined between the upstanding pieces 12, 13, in order for the control current flowing through the coil 8b to be accurately perpendicular to the interlinking magnetic flux radiated between the upstanding pieces 12, 13. For accurately generating the driving force for the objective lens 1 in the planar direction perpendicular to the optical axis, the center between the linear portion 19a, 19b of the rectangular coil portions 9b, 9c of the tracking coil 9 needs to be coincident with a line X--X' in FIG. 4 which is the center along the width of the upstanding pieces 12, 13, in order for the control current flowing through the linear portion 19a, 19b to be accurately perpendicular to the interlinking magnetic flux radiated between the upstanding pieces 12, 13. On the other hand, for generating the uniform driving force in a direction parallel to the optical axis and in a planar direction normal to the optical axis, the center of a line Z--Z' in FIG. 5 extending in a direction along the height of the coil portion 8b of the focusing coil 8 and the linear portions 19a, 19b of the tracking coil 19, needs to be coincident with the center along the height of the magnet 14.
By arranging the focusing coil 8 and the tracking coil 9 relative to the magnetic circuit 4 as described above, the center of generation of the diving force generated by the interaction of the control current supplied to the focusing coil 8 and the tracking coil 9 and the magnetic flux radiated from the magnet 14 for displacing the object lens 1 in the directions parallel and orthogonal to its optical axis is disposed at a point of intersection of the lines Y--Y' and X--X' in FIG. 4 and the line Z--Z' in FIG. 5.
By coinciding the center of gravity P of the bobbin 2 as a movable part carrying the objective lens 1 with the point of intersection of the lines Y--Y' and X--X' in FIG. 4 and the line Z--Z' in FIG. 5, the bobbin 2 is displaced in the directions parallel to and at right angles to the optical axis of the objective lens 1 with high response characteristics without producing distortion relative to the driving force parallel to or normal to the optical axis of the objective lens 1. Since the bobbin 2 is displaced with high response characteristics without producing distortion etc., the objective lens 1 mounted on the bobbin 2 is displaced correctly in the direction parallel and normal to the optical axis of the objective lens 1 responsive to the control current supplied to the focusing coil 8 and the tracking coil 9, respectively.
The objective lens 1 is mounted so that its optical axis is disposed on the line X--X in FIG. 4 and parallel to the line Z--Z' in FIG. 5.
Meanwhile, the focusing coil 8 and the tracking coil 9, employed for the conventional objective lens driving device, are of a three-dimensional structure comprising a sole wire wound in a tube or rectangle. This renders it extremely difficult to form the focusing coil 8 and the tracking coil 9 employed in the objective lens driving device so as to be of a unified constant size. If the focusing coil 8 and the tracing coil 9, having size variation, are mounted on the bobbin 2, it becomes impossible to render the center of gravity P of the bobbin 2 inclusive of the objective lens 1 constant. Above all, if there is any variation in shape or size of the focusing coil 8 or the tracking coil 9 in portions thereof spaced apart from the center of gravity P of the bobbin 2, inclusive of the objective lens 1, when the focusing coil 8 or the tracking coil 9 is mounted in position, such variation affects the position of the center of gravity P of the bobbin 2 inclusive of the objective lens 1 seriously.
In addition, since the focusing coil 8 carrying the tracking coil 9 is mounted within the aperture 7 of the bobbin 2 using an adhesive, it is difficult to mount the focusing coil on the bobbin 2, while it is extremely difficult to mount it with high mounting accuracy relative to the bobbin 2. Thus the position of the center of gravity P of the bobbin 2 inclusive of the objective lens 1 becomes different from one object lens driving device to another, while it becomes difficult to set the position of the center of gravity P accurately.
The bobbin 2 is formed as a molded article of a synthetic material, while the focusing coil 8a and the tracking coil 9 are formed by copper wires. The specific gravity of the synthetic material constituting the bobbin 2 is on the order of 1.5, while that of the copper wire of the focusing coil 8 or the tracking coil 9 is 8.9. Consequently, should there be any variation in the shape or size of the focusing coil 8 or the tracking coil 9 or in the mounting position thereof relative to the bobbin 2, it becomes impossible to set the position of the center of gravity P of the bobbin 2 as the movable part, inclusive of the objective lens 1, without size or shape variance from one object lens driving device to another.
If the position of the center of gravity P of the movable part is not constant from one object lens driving device to another, as described above, the center of generation of the driving force generated by the interaction between the focusing coil 8 or the tracking coil 9 and the magnetic circuit 9 is not coincident with the center of gravity P of the movable part with the result that the objective lens 1 cannot be displaced in the directions normal and parallel to its optical axis with high response to the control current without producing the force of deviation such as distortion. The optical beam radiated via the objective lens 1 on the signal recording surface of the optical disc cannot be focusing and tracking controlled with high accuracy so that information signals cannot be recorded or reproduced with high recording and/or reproducing characteristics.
The above-described object lens driving device is so designed that the coil portion 8b on one lateral side of the rectangular tubular wound focusing coil 8 is inserted into a space between the upstanding pieces 12 and 13 of the yoke 5, as shown in FIGS. 1 and 6. Consequently, a coil portion 8c of the focusing coil 8 opposite to the coil portion 8b inserted between the upstanding pieces 12, 13 faces the upstanding piece 12 fitted with the magnet 14. When the focusing coil 8 is fed with the control current, there is produced, in addition to the driving force f.sub.1 generated by the interaction between the control current and one half side of the coil portion 8b interlinked with the effective magnetic flux Bg radiated from the magnet 14 into a space between the upstanding pieces 12 and 13, a driving force f.sub.2 generated by the interaction between the control current and the other half side of the coil portion 8b interlinked with the effective magnetic flux Bg' radiated from the magnet 14 towards the back surface of the upstanding piece 12, as shown in FIG. 8. This driving force f.sub.2, generated by the interaction between the control current and the opposite one-half side of the coil portion 8b interlinked with the stray magnetic flux Bg', acts in the opposite direction to the driving force f.sub.1 generated by the interaction between the control current and the one-half side of the coil portion 8b interlinked with the effective magnetic flux Bg, and acts for canceling the driving force driving the objective lens 1 along the optical axis, such that the driving force driving the objective lens 1 along the optical axis cannot be exploited effectively.
Thus, with the conventional objective lens driving device, shielding means, such as a shield plate, is provided for shielding the stray magnetic flux, or the focusing coil 8 is increased in size, in order to eliminate the effect of the stray flux. However, provision of the shielding means or the use of the large-sized focusing coil 8 leads to an increased size of the objective lens driving device itself.
The portion of the focusing coil 8 of the conventional objective lens driving device which is effective to generate the driving force driving the objective lens 1 along the optical axis in cooperation with the magnetic flux radiated from the magnet 14 is solely the above-mentioned one-half side of the coil portion 8b of the focusing coil 8 inserted between the upstanding pieces 12, 13, shown shaded in FIG. 6. On the other hand, the portion of the focusing coil 8 of the conventional objective lens driving device which is effective to generate the driving force driving the objective lens 1 in the planar direction normal to the optical axis in cooperation with the magnetic flux radiated from the magnet 14 is solely the linear portions 19a, 19b of the coil portions 9b, 9c of the tracking coil 19 facing the magnet 14, shown shaded in FIG. 7. That is, the portions of the focusing coil 8 and the tracking coil 9 effective to produce the driving force driving the objective lens 1 account for about one-fourth of the entire coils, thus the exploiting efficiency being extremely low. Due to the low exploitation efficiency of the focusing coil 8 and the tracking coil 9, more current is needed for displacing the objective lens 1, resulting in increased heat emission from the objective lens driving device. Such heat emission affects the operation of the semiconductor laser as the light source of the optical pickup device, resulting in obstruction to stable light beam oscillation.
In addition, the focusing coil 8 employed in the conventional objective lens driving device is a tubular wound coil and hence tends to be increased in self-inductance. Besides, the upstanding portion 12 of the yoke 12 of the magnetic circuit 4 is inserted into the tubular focusing coil 8, the upstanding piece 12 acts as an iron core for further increasing the self-inductance of the focusing coil 8. Should the self-inductance of the focusing coil 8 be increased in this manner, the phase rotation is increased acutely beyond 180.degree. in the high frequency range of the focusing error signal when the driving current corresponding to the focusing error signal is fed to the focusing coil 8 via the driving control circuit for displacing the objective lens 1, with the consequence that focusing control corresponding to the focusing error signal becomes infeasible. For avoiding such focusing control infeasibility, electrical phase correction is carried out on the side of the control circuit detecting the focusing error signal for supplying the control circuit to the focusing coil 8. Should the phase correction value be increased, high harmonic components of the driving current supplied to the focusing coil 8 are increased in proportion to the correction value, thereby increasing the power consumption. Should the power consumption be increased, the operation of the semiconductor laser constituting the optical pickup device tends to be unstable.